Current ultrasonic imaging systems make use of contrast agents in circulation to enhance ultrasound returns. Contrast agents are substances which strongly interact with ultrasound waves and return echoes which may be clearly distinguished from those returned by blood and tissue. Microbubbles are currently employed as a contrast agent and provide a non-linear behavior in certain acoustic fields. Such behavior is readily detectable by use of known algorithms. Microbubble contrast agents are useful for imaging of the body's vascular system and are injectable through the veins and arteries. They are subsequently filtered from the blood stream by the lungs, kidneys and liver.
Microbubble contrast agents generally comprise coated gas bubbles that are stable in the body for a significant period of time. The coating shells serve to protect the gas from diffusion into the blood stream. At moderately high ultrasound pressure amplitudes, the shells of the microbubbles can be caused to rupture, freeing the internal gas and substantially eliminating the detectability thereof by incident ultrasound waves.
U.S. Pat. No. 5,833,613 to Averkiou et al. discloses an ultrasound method for imaging of contrast agents. In one embodiment, a rate of re-perfusion of an anatomical region is accomplished by initially destroying the contrast agent within the region, and then subsequently imaging the region to determine the rate of re-insertion of the contrast agent. The Averkiou et al. method of indicating the rate of re-perfusion utilizes plotted curves that indicate echo returns from interrogating ultrasound beams. Initially, Averkiou et al. transmit high energy ultrasound pulses to destroy the microbubbles in the region to be imaged. A short time later, lower energy, imaging, ultrasound pulses are transmitted again, the echoes received and imaged to measure the degree of microbubble re-infusion by, for example, counting or integrating the pixels in the area which show re-infused microbubbles. The measure of the number of re-infused microbubbles in the region is plotted in curve format. Non-destructive pulses can thereafter be repetitively transmitted and echoes received and plotted as a sequence of points to indicate the rate of re-perfusion.
The method of displaying the re-perfusion rate taught by Averkiou et al. does not provide the user with a display that is directly related to the area being imaged. Rather, the user is required to associate an image of the area being re-perfused with the curve presentation. Analysis of the plotted re-perfusion curve and its association with the region being viewed requires training and experience on the part of the user.
U.S. Pat. No. 5,860,931 to Chandler teaches a further method for measuring perfusion in an ultrasound system. Concentration levels of a contrast agent are measured in a first region both before and after reducing the concentration level of contrast agent in a second region. The position of the second region overlaps the position of the first region such that any dimension of the second region differs from any corresponding dimension of the first region by less than a factor of two. This method enables allows contrast agent flow to be calculated.
There is a need for an improved method for displaying to the user a rate of re-perfusion of an anatomical region. Such a display should enable the user to directly correlate the rate of re-perfusion with areas of anatomy so as to enable the user to discern which areas exhibit abnormal rates.